1. Field of the Invention
The present invention relates to an air bag starter that is installed in a vehicle and protects a driver and a passenger inside the vehicle from a shock at the time an accident, such as collision, occurs and to a backup circuit that is used in the air bag starter to accurately back up an inflating operation of the air bag.
2. Description of Related Art
A so-called air bag system that is installed in a vehicle (e.g., a car) and inflated by, for example, igniting a powder loaded in the system or breaking a plug (stopper) of a high-pressure cylinder upon occurrence of a collision or the like, in order to protect a driver and a passenger inside the vehicle from a shock, has been put into practical use. A system for ignition of the air bag including an electric circuit is hereinafter referred to as an air bag starter or simply a starter.
FIG. 5 shows an example of a construction of a conventional starter, for example, a starter disclosed in the International Application No. PCT/JPOO/07051. Referring to FIG. 5, reference numeral 1 is a battery installed in a vehicle, and numeral 2 is an ignition switch for starting an engine of the vehicle. Numeral 3 is a driver's seat air bag disposed in the driver's seat, and numeral 4 is an igniter for a first-stage inflator (detonating powder or the like not shown in the drawing) for inflating the driver's seat air bag 3. Numeral 5 is an igniter for a second-stage inflator that is ignited, when required, synchronously with the first-stage inflator or with a predetermined delay after the ignition of the first-stage inflator.
Numeral 6 is an assistant driver's seat air bag disposed in the assistant driver's seat, and numeral 7 is an igniter for a first-stage inflator for inflating the assistant driver's seat air bag 6. Numeral 8 is an igniter for a second-stage inflator (detonating powder or the like not shown) that is ignited, when required, synchronously with the first-stage inflator or with a predetermined delay after the ignition of the first-stage inflator.
Numeral 9 is an air bag control unit including an electric circuit for transmitting an electric signal to each of the foregoing igniters and igniting them. Numeral 10 is a DC-to-DC converter boosting input voltage supplied from the battery 1 installed in the vehicle and outputting the voltage, and numeral 11 is a backup condenser (feeding means) charged with a power outputted by the DC-to-DC converter 10. Numeral 12 is a mechanical acceleration switch arranged to close when a decelerating acceleration of the vehicle exceeds a predetermined level. The mechanical acceleration switch 12 is arranged so that an electric current is not sent to the inflators owing to any malfunction of any driving transistor and prevents the inflators from ignition caused by the malfunction. Such a mechanical acceleration switch 12 is disclosed in, for example, the Japanese Patent Publication (unexamined) No. 211023/1997. The mechanical acceleration switch disclosed in this Japanese Patent Publication (unexamined) No. 211023/1997 has a construction in which the switch is closed upon occurring a weight shift owing to so-called a colliding acceleration at the time of a collision, and the switch is opened upon returning the weight to a predetermined position by spring force when the acceleration disappears. Numeral 13 is an acceleration sensor that measures an acceleration of the vehicle and outputs a signal conforming to the acceleration. Numeral 14 is ignition-judging means that makes a judgment on ignition of the first-stage inflator and the second-stage inflator of the driver's seat air bag 3 as well as the first-stage inflator and the second-stage inflator of the assistant driver's seat air bag 6 on the basis of the acceleration signal inputted from the acceleration sensor 13. This ignition judging means 14 turns on related driving transistors (described later) and at the same time outputs a signal X (shown in FIG. 5) synchronizing with “on” of the driving transistor.
Numeral 15 is a second-stage forced igniting means (forced igniting means) that is disposed in parallel to the mechanical acceleration switch 12, connects the backup condenser 11 with a common connecting portion where the first-stage igniters 4 and 7 and the second-stage igniters 5 and 8 are connected, and supplies an ignition current to the second-stage igniters 5 and 8 at the time of forcedly igniting the second-stage inflators in response to the signal from the ignition judging means 14. Numeral 31 is a driving transistor (switching means for forced ignition or a switch) disposed in parallel to the mechanical acceleration switch 12 between the backup condenser 11 and the common connecting portion where the first-stage igniters 4 and 7 and the second-stage igniters 5 and 8 are connected in parallel.
Numeral 32 is closure detecting means that detects the mechanical acceleration switch 12 being closed and holds a closure signal (hereinafter referred to as signal A for convenience of explanation) output for a predetermined time after the mechanical acceleration switch 12 is once closed and then opened. Numeral 33 is a two-input AND gate (logic means) in which the two input terminals are connected to the ignition judging means 14 and to (the signal A of) the closure detecting means 32 respectively, and an output terminal is connected to a gate (control terminal) of the driving transistor 31.
The driving transistor 31, the closure detecting means 32, and the AND gate 33 constitute the second-stage forced igniting means 15. Numerals 16 and 17 are driving transistors (first switching means) that control the turning on and off of an electric current for igniting the first-stage inflator of the driver's seat air bag 3. Numerals 18 and 19 are driving transistors (second switching means) for the second-stage inflator of the driver's seat air bag 3. Numerals 20, and 21 are driving transistors (first switching means) for the first-stage inflator of the assistant driver's seat air bag 6. Numerals 22 and 23 are driving transistors (second switching means) for the second-stage inflator of the assistant driver's seat air bag 6. The ignition judging means 14 is connected to gates (control terminals) of the driving transistors 16, 17, 18, 19, 20, 21, 22 and 23, and controls “on” and “off” of the driving transistors 16, 17, 18, 19, 20, 21, 22 and 23 in response to the acceleration signal inputted from the acceleration sensor 13.
Now, described below are operations of the conventional air bag system shown in FIG. 5. FIGS. 6 and 7 are timing charts showing various aspects of an igniting operation in the air bag system shown in FIG. 5.
When a key (not shown) is turned to a position for starting the engine and the ignition switch 2 is closed, the DC-to-DC converter boosts a direct-current power outputted from the vehicle-mounted battery 1, and the backup condenser 11 is charged with the power.
In the case of a collision, when a decelerating acceleration caused by the collision exceeds a predetermined acceleration, the mechanical acceleration switch 12 is closed. In this process, the closure detecting means 32 detects the mechanical acceleration switch 12 being closed, converts the output signal (the signal A) delivered to the AND gate 33 from an L (i.e., low) level to an H (i.e., high) level, and holds the signal level.
On the basis of the acceleration signal inputted from the acceleration sensor 13, the ignition judging means 14 chooses a proper inflation form from among four forms of the driver's seat air bag 3 and the assistant driver's seat air bag 6 described below.
Inflation form A: the air bags 3 and 6 are not inflated.
Inflation form B: only the first-stage inflators are ignited, and the air bags 3 and 6 are inflated to a proper degree.
Inflation form C: The first-stage inflators are ignited and then the second-stage inflators are ignited after a predetermined delay, such that the air bags 3 and 6 are inflated strongly to a certain degree.
Inflation form D: The first-stage inflators and the second-stage inflators are ignited at the same time, such that the air bags 3 and 6 are inflated strongly.
If either the inflation form B or C is chosen, the ignition judging means 14 turns on the driving transistors 16, 17, 20 and 21 in order to ignite the first-stage inflators of the driver's seat air bag 3 and the assistant driver's seat air bag 6. As a result, an electric current necessary for the ignition is conducted to the first-stage igniters 4 and 7 to ignite the first-stage inflators, such that the driver's seat air bag 3 and the assistant driver's seat air bag 6 are inflated to the proper degree. In this case, as shown in FIG. 6, the mechanical acceleration switch 12 is supposed to be closed at the time when the driving transistors for igniting the first-stage inflators are turned on, and it is therefore possible to ignite the first-stage inflators.
Next, if choosing the inflation form B, since for reasons of safety it is preferable that the second-stage inflators not be left without igniting them, the second-stage inflators are forcedly ignited immediately after the collision. In this case, a time lag from the ignition of the first-stage inflators to the ignition of the second-stage inflators is, for example, T or T′=100 milliseconds. As shown in FIG. 6, whether the mechanical acceleration switch 12 is opened or closed is not always clear at the time when the second-stage inflators are ignited. To cope with this, when the driving transistors 18, 19, 22 and 23 are turned on, the output signal X, which is input to the AND gate 33, is converted from an L level to an H level at the same time. The two signals having the H level (i.e., the signal A and the signal X) are inputted to the two input terminals of the AND gate 33 respectively and, thus, a signal having the H level is outputted from the AND gate 33 such that the driving transistor 31 is turned on. As a result, the common connecting portion where the first-stage igniters 4 and 7 and the second-stage ignites 5 and 8 are connected in parallel and the backup condenser 11 are electrically connected, and therefore the backup condenser 11 supplies a current to the second-stage igniters 5 and 8 such that the second-stage inflators are ignited.
According to the foregoing construction, when the signal X that commands the second-stage inflators to ignite is outputted by the ignition judging means 14, the signal A indicating that the mechanical acceleration switch 12 is closed is also outputted by the closure detecting means 32. As a result, the driving transistor 31 is turned on and the second-stage igniters 5 and 8 are ignited.
However, a collision involving a vehicle includes extremely indefinite factors, and therefore an operation waveform of the mechanical acceleration switch 12 is not always stable, contrary to the waveform illustrated in FIG. 6. FIG. 7 is a timing explanation diagram for explaining a timing and shows a waveform that is more likely to actually occur. At the moment of collision, the mechanical acceleration switch 12 presents a so-called chattering phenomenon such as an irregular vibration. As a result, a first-stage ignition current delivered from the condenser 11 to the air bag 3 or 6 has a waveform that oscillates as indicated by numeral 99 in FIG. 7, and, for example, an average value of the electric current is lowered and the air bag ignition becomes uncertain, eventually resulting in a reduced reliability of operation.
To overcome the aforementioned disadvantage in the conventional air bag starter, in addition to adopting any arrangement that reduces the chattering in the mechanical acceleration switch 12, it is necessary to improve the reliability of the ignition operation by making the time for turning on the first-stage ignition driving transistors 16, 17, 20 and 21 longer than a minimum time necessary for the ignition of the inflators and enlarging a capacity of the condenser 11 so that the current is applied there from for a longer time. Alternatively, it is necessary to prevent the negative influence of chattering by boosting a voltage for charging the condenser 11 and shortening a rise time of the current.
The foregoing explanation describes a two-stage ignition type air bag system, and it is a matter of course that the same problems may occur in a one-stage ignition type air bag system. The one-stage ignition type air bag system is not provided with the forced igniting means 15 in FIG. 5, and the air bags 3 and 6 are not provided with any portion related to the second-stage inflators. However, the operation for ignition of the first-stage igniters 4 and 7 is the same, and this system likewise has the foregoing problems caused by chattering of the mechanical acceleration switch 12 in the same manner as in the case of the two-stage ignition type air bag system.
As described above, in the conventional air bag starter, if any chattering takes place in the mechanical acceleration switch 12 at the time of turning on the driving transistor and delivering the ignition current, the ignition current is not sufficiently delivered to the driver's seat air bag 3 and/or the assistant driver's seat air bag 6 and, in some cases, the inflators do not ignite. As a result, a problem exists in that the reliability of operation is lowered.